Process for producing aluminum alloy substrate for lithographic printing plate

ABSTRACT

A process for producing an aluminum alloy substrate for a lithographic printing plate is provided, wherein an excellent grained surface can be realized even when the heat treatment time and alkali etching are shortened. The method includes the steps of preparing an ingot composed of specific ranges of Fe, Si, Cu, Ti, B, and unavoidable impurities; subjecting the ingot to a homogenization treatment composed of the first stage of holding at 510° C. to 560° C. for 30 minutes to 2 hours and the latter stage of holding at 460° C. to 500° C. for 30 minutes to 2 hours; starting hot rolling, followed by finishing at 360° C. or more; conducting cold rolling; conducting intermediate annealing at a heating temperature of X° C. for a holding time of Y sec in an inert gas atmosphere, where X is 400° C. to 620° C. and Y≧2×10 8 ×exp(−0.0284X) is satisfied; and conducting final cold rolling.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for producing an aluminumalloy substrate for a lithographic printing plate, wherein anelectrochemically grained surface on the level better than or equal tothe levels of known grained surfaces can be realized even when alkalietching corresponding to a pretreatment of a surface-graining treatmentis shortened.

2. Description of the Related Art

Generally, a JIS 1000 series aluminum alloy sheet of 0.1 to 0.5 mm inthickness has been used as an aluminum alloy substrate for alithographic printing plate. Such an aluminum alloy sheet has usuallybeen produced by scalping an ingot prepared by a semicontinuous castingmethod so that the surface is removed, subjecting the scalped ingot to ahomogenization treatment, and subjecting the homogenized ingot to hotrolling, cold rolling, intermediate annealing, and final cold rolling.

The thus produced aluminum alloy substrate for the lithographic printingplate is subjected to a surface-graining treatment through a stepcomposed of either one of or a combination of at least two of amechanical method, a chemical method, and an electrochemical method. Theresulting aluminum alloy substrate is further subjected to ananodization treatment, and optionally to a hydrophilic treatment to givea lithographic printing plate support. In addition, the support iscoated with a photosensitive material to form a photosensitive layer. Ifnecessary, the photosensitive layer is strengthened by a heating-burningtreatment, so that a photosensitive lithographic printing plate isprepared.

The resulting lithographic printing plate substrate is subjected to atreatment for producing the lithographic printing plate, in which imageexposure, development, water washing, lacquering, and the like areconducted sequentially, to give a printing original plate. Thephotosensitive layer remaining still undissolved after theabove-described development is water repellent, and forms image areasserving as an ink-accepting portion which selectively accepts ink alone.The surface of the aluminum alloy support under the photosensitive layeris exposed at the portions where the photosensitive layer is dissolved,and the portions form nonimage areas serving as water-accepting portionsdue to the hydrophilic property thereof. In this development treatment,the surface of the aluminum alloy support under the photosensitive layermust have a water retention property in order to form a nonimage areaserving as a water-accepting portion. Consequently, the surface is madeto be a grained surface having uniform asperities by a surface-grainingtreatment.

In recent years, energy conservation and improvement of productivityhave been required particularly in the method for producing alithographic printing plate.

In order to achieve the above-described energy conservation andimprovement of productivity, methods for producing a substrate for alithographic printing plate have been proposed, in which the time ofhomogenization treatment of an ingot is shortened, or the time ofintermediate annealing treatment is shortened in the cold rolling.However, in each case, the surface of the substrate is dissolved andremoved by a long-duration alkali etching corresponding to apretreatment of a surface-graining treatment. Therefore, theseproduction methods are not satisfactory.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide aprocess for producing an aluminum alloy substrate for a lithographicprinting plate, wherein a grained surface on the level better than orequal to the levels of known substrates can be realized even when theheat treatment time of an aluminum alloy plate is shortened or alkalietching corresponding to a pretreatment of a surface-graining treatmentis shortened.

In order to achieve the above-described object, a process for producingan aluminum alloy substrate for a lithographic printing plate, accordingto the present invention, is characterized by including the steps ofpreparing an aluminum alloy ingot having a JIS 1000 series composition;subjecting the aluminum alloy ingot to a homogenization treatmentcomposed of the first stage of holding at 510° C. to 560° C. for 30minutes to 2 hours and the latter stage of holding at 460° C. to 500° C.for 30 minutes to 2 hours; starting hot rolling and, thereafter,finishing the hot rolling at a temperature of 360° C. or more;conducting cold rolling; conducting intermediate annealing at a heatingtemperature of X° C. for a holding time of Y sec in an inert gasatmosphere, where X is 400° C. to 620° C. and Y≧2×10⁸×exp(−0.0284X) issatisfied; and conducting final cold rolling.

It is desirable that the holding time of the homogenization treatment ofthe above-described ingot is 1 hour or less. This treatment homogenizessimple metallic substances, e.g., Si, Fe, and Cu, or compounds thereofwhich have been segregated during the casting and have not beenadequately homogenized.

Preferably, the inert gas atmosphere of the above-described intermediateannealing treatment is composed of a combustion gas produced bycombustion of an inexpensive hydrocarbon gas, e.g., a propane gas and abutane gas, followed by dehydration.

Preferably, the above-described aluminum alloy ingot has a compositioncomposed of 0.1 to 0.50 percent by weight of Fe, 0.03 to 0.30 percent byweight of Si, 0.002 to 0.040 percent by weight of Cu, 0.01 to 0.05percent by weight of Ti, 0.0001 to 0.02 percent by weight of B, andsubstantially the balance of aluminum.

According to the present invention, the aluminum alloy ingot having theabove-described composition is used, and the homogenization, the hotrolling, the cold rolling, the intermediate annealing, and the finalcold rolling are conducted on the conditions in the above-describedranges. Consequently, an aluminum alloy substrate for a lithographicprinting plate provided with a grained surface on the level better thanor equal to the levels of known substrates can be realized even when theheat treatment time and the pretreatment time of the surface-grainingare shortened.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph showing the ranges of a heating temperature and aholding time, which are conditions of the intermediate annealing of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The aluminum alloy in the present invention is a JIS 1000 series alloy.This has a composition suitable for exerting the effect on formingintermetallic compounds of elements, e.g., Fe, Si, Ti, and B, theintermetallic compounds providing strength and electrochemically formingan uniform roughened surface.

In particular, the above-described effect is exerted excellently when Feis specified to be 0.1 to 0.50 percent by weight, Si is specified to be0.03 to 0.30 percent by weight, Cu is specified to be 0.002 to 0.040percent by weight, Ti is specified to be 0.01 to 0.05 percent by weight,and B is specified to be 0.0001 to 0.02 percent by weight.

Elements, e.g., Mg, Mn, Cr, Zr, V, Zn, Ni, Ga, Li, and Be, originatedfrom a base metal and return scraps in the preparation of a melt may beincluded as unavoidable impurities. However, when the content is such avery small quantity as specified with respect to JIS 1000 series, theeffect of the present invention is not adversely affected to a largeextent.

The aluminum alloy substrate for a lithographic printing plate in thepresent invention is produced as described below.

A melt of an aluminum alloy having the above-described composition andprepared by conducting treatments, e.g., degassing and slag removal, isconventionally cast into an ingot. The casting method is notspecifically limited, and examples of casting methods include acontinuous casting method and a semicontinuous casting method. However,the semicontinuous casting method is desirable. The thickness of theingot produced by the semicontinuous casting method is not specificallylimited. However, the thickness is usually about 500 to 600 mm.

After the surface of the ingot is scalped, a homogenization treatmentcomposed of the first stage of holding at 510° C. to 560° C. for 30minutes to 2 hours and the latter stage of holding at 460° C. to 500° C.for 30 minutes to 2 hours is conducted.

The heating at 510° C. to 560° C. in the first stage of thehomogenization treatment is directed to homogenize elements in theingot. If the temperature is lower than the lower limit, theabove-described effect is reduced, and if the temperature exceeds theupper limit, the thickness of an oxide coating is increased. Therefore,these are not preferable. The holding time is 30 minutes to 2 hours. Ifthe holding time is less than the lower limit, the homogenization effectis reduced, and if the holding time exceeds the upper limit, thethickness of an oxide coating is increased. Therefore, these are notpreferable. Preferably, the holding time is 1 hour or less.

The heating at 460° C. to 500° C. in the latter stage is directed toregulate the formation of an oxide coating and to control thetemperature of the ingot at a low temperature such that occurrence ofrecrystallization in the rolled plate is suppressed during the hotrolling and recrystallization is effected to realize a finerecrystallization structure when the hot rolling is finished. If thetemperature is lower than the lower limit, the plate temperature becomestoo low when the hot rolling is finished and, thereby, it is difficultto realize the fine recrystallization structure. If the temperatureexceeds the upper limit, recrystallization occurs in the rolled plateduring the hot rolling and, thereby, it is difficult to realize the finerecrystallization structure. The holding time is 30 minutes to 2 hours.If the holding time is less than the lower limit, the temperature of theingot is significantly varied depending on positions of the ingot and,thereby, it is difficult to realize the fine recrystallization structureall over the surface layer of the rolled plate. If the holding timeexceeds the upper limit, the thickness of an oxide coating is increased.Therefore, these are not preferable. Preferably, the holding time is 1hour or less.

After the homogenization treatment, the hot rolling is conductedgenerally in at least several rolling passes. The hot rolling is startedat the ingot temperature of 460° C. to 500° C., that is, the temperatureof the homogenization treatment, and the hot rolling is finished at atemperature of 360° C. or more to effect recrystallization. Preferably,the thickness of the hot-rolled plate is 2 to 10 mm. The rollingreduction rate in the final pass of the hot rolling is desirably atleast 55%. This rolling reduction rate causes a large strain in thesurface layer of the hot-rolled plate, and a fine recrystallizationstructure having an average recrystallized grain size of less than 150μm and a maximum grain size of less than 200 μm in a direction normal tothe rolling direction can easily be realized at least in the surfacelayer of the hot-rolled plate.

In this specification, the surface layer of the hot-rolled plate refersto a region at depths ranging from 200 μm to 800 μm from the surface inthe case where the thickness of the hot-rolled plate is 2 to 10 mm.Here, as for the region at depths ranging up to 200 μm from the surface,this value of the depth is in terms of a depth of the hot-rolled plateand corresponds to the depth to be removed by the etching treatmentconducted as a pretreatment of the surface-roughening treatment and thedepth to be removed by the electrochemical surface-roughening treatmentand the like when the hot-rolled plate is finally formed into analuminum alloy substrate having a thickness of about 0.15 to 0.5 mm bythe cold rolling.

The hot-rolled plate having the fine recrystallization structure aftercompletion of the hot rolling is cooled and cold-rolled. Theintermediate annealing treatment after the cold rolling is conducted ata heating temperature of X° C. for a holding time of Y sec in an inertgas atmosphere, where X is 400° C. to 620° C. and Y≧2×10⁸×exp(−0.0284X)is satisfied. This treatment is conducted in order to softening thecold-rolled plate having been hardened in cold processing, to realize afine recrystallization structure, and to homogenize simple metallicsubstances, e.g., Si, Fe, and Cu, or compounds thereof which have notbeen adequately homogenized in the homogenization treatment or whichhave been made heterogeneous during the hot rolling.

The relationship between the heating temperature and the holding time ofthe intermediate annealing will be described with reference to FIG. 1.

In FIG. 1, the vertical axis represents the holding time Y (unit:second, a log scale), and the horizontal axis represents the heatingtemperature X (unit: ° C.). The straight line is represented byY=2×10⁸×exp(−0.0284X), and indicates a boundary line between the rangein which a uniform roughened surface is realized and a range in which anunetched portion results. The boundary line was determined based on manyexperiments in which the intermediate annealing treatment was conductedin an inert gas atmosphere, the substrate prepared by cold rolling afterthe intermediate annealing treatment was subjected to alkali etching for10 seconds and, thereafter, a surface-graining treatment was conducted.A uniform roughened surface can be realized on the condition in the sideof the holding time longer than or equal to that indicated by thisstraight line (the upper side in the drawing). On the other hand, anunetched portion results on the condition in the side of the holdingtime shorter than that indicated by this straight line (the lower sidein the drawing).

If the heating temperature is lower than 400° C., a glossy unetchedportion results, and if the heating temperature exceeds 620° C., growthof crystal grains is advanced and, thereby, it becomes difficult torealize fine recrystallization.

This intermediate annealing treatment is directed to reduce thethickness of an oxide coating formed on the plate surface by treating inthe inert gas atmosphere. The inert gas atmosphere of this intermediateannealing treatment may be composed of an argon gas or a nitrogen gas aslong as the gas has a low dew point, for example, the dew point is about−40° C. to −10° C. A combustion gas of hydrocarbon gas, e.g., a propanegas and a butane gas, is preferable because these are inexpensive, ifwater in the combustion gas is removed. After the intermediate annealingis conducted, final cold rolling is conducted to attain a desiredthickness, for example, up to a thickness of 0.15 to 0.5 mm, so that analuminum alloy substrate for a printing plate is produced.

EXAMPLES

Examples of the Present Invention

Each aluminum alloy melt was prepared, and a slab having a thickness of530 mm was cast by a semicontinuous casting method. The composition ofthe slab is shown in Table 1. Both surfaces of the slab were scalped sothat the thickness was decreased by 15 mm per side. A homogenizationtreatment was conducted by heating and holding at 540° C. for 1 hourand, thereafter, heating and holding at 480° C. for 1 hour.Subsequently, the slab was taken out of a homogenization furnace, andhot rolling was started at a temperature of the homogenizationtreatment. The rolling reduction rate in the final pass of the hotrolling was 90%, the thickness of the rolled plate was 7 mm, and thetemperature was 380° C. at the finish of the hot rolling. After coolingwas conducted, cold rolling was conducted up to the thickness of 1 mm.Intermediate annealing was conducted on various conditions and,thereafter, final cold rolling was conducted up to the thickness of 0.3mm, so that an aluminum alloy substrate was produced. In theintermediate annealing, a nitrogen gas having a dew point of −40° C. wasused for constituting the treatment atmosphere. The resulting substratewas taken as the sample of the present invention.

Comparative Examples

In the condition of preparation of the above-described samples of thepresent invention, the holding time was set at 1 hour or 12 hours,whereas the temperature of the condition of the homogenization treatmentwas the same as that of the present invention. Intermediate annealingwas conducted on various conditions. The intermediate annealing wasconducted in an atmosphere of a nitrogen gas or in an atmosphere of airas in Examples of the present invention, so that samples for comparisonwere prepared. TABLE 1 Alloy No. Fe Si Cu Ti B Remainder Remarks 1 0.320.10 0.020 0.02 0.001 Al and Example of impurities alloy of theinvention 2 0.32 0.07 0.003 0.015 0.001 Al and Example of impuritiesalloy of the invention

Each sample of the above-described Examples of the present invention andComparative examples was immersed in 10% NaOH aqueous solution at atemperature of 50° C. for 10 seconds. Electrochemical surface grainingwas conducted through electrolytic etching at an anodic electricityquantity of 150 Coulomb/dm² in 1% nitric acid aqueous solution at 40° C.by using a power supply having an electrolytic waveform with alternatingpolarity. The treated sample was cleaned in sulfuric acid and,thereafter, uniformity of asperities of the roughened surface and glossyportions were observed with SEM. As for the glossy portions, a portionhaving a maximum diameter of 0.5 mm or more was evaluated as an unetchedportion. The results are shown in Table 2. TABLE 2 Intermediate Thenumber Homogenization annealing Atmosphere of Alkali of unetched SampleAlloy treatment condition condition intermediate etching portions No.No. ° C. × hour ° C. × second annealing second number/m² Remarks 1 1 540× 1, then 480 × 1 400 × 3600 nitrogen gas 10 0 Example of the invention2 1 540 × 1, then 480 × 1 440 × 900  nitrogen gas 10 0 Example of theinvention 3 1 540 × 1, then 480 × 1 450 × 900  nitrogen gas 10 0 Exampleof the invention 4 1 540 × 1, then 480 × 1 480 × 3600 nitrogen gas 10 0Example of the invention 5 1 540 × 1, then 480 × 1 500 × 200  nitrogengas 10 0 Example of the invention 6 1 540 × 1, then 480 × 1 530 × 100 nitrogen gas 10 0 Example of the invention 7 1 540 × 1, then 480 × 1 550× 50  nitrogen gas 10 0 Example of the invention 8 1 540 × 1, then 480 ×1 590 × 15  nitrogen gas 10 0 Example of the invention 9 1 540 × 1, then480 × 1 370 × 7200 nitrogen gas 10 7 Comparative example 10 1 540 × 1,then 480 × 1 400 × 900  nitrogen gas 10 6 Comparative example 11 1 540 ×1, then 480 × 1 450 × 120  nitrogen gas 10 10 Comparative example 12 1540 × 1, then 480 × 1 550 × 10  nitrogen gas 10 5 Comparative example 131 540 × 1, then 480 × 1 480 × 3600 air 10 ⋆ Comparative example 14 1 540× 1, then 480 × 1 550 × 60  air 10 ⋆ Comparative example 15 1 540 × 12,then 480 × 1 480 × 3600 air 10 ⋆ Comparative example 16 2 540 × 1, then480 × 1 420 × 4800 nitrogen gas 10 0 Example of the invention 17 2 540 ×1, then 480 × 1 450 × 1000 nitrogen gas 10 0 Example of the invention 182 540 × 1, then 480 × 1 480 × 900  nitrogen gas 10 0 Example of theinvention 19 2 540 × 1, then 480 × 1 500 × 3600 nitrogen gas 10 0Example of the invention 20 2 540 × 1, then 480 × 1 520 × 200  nitrogengas 10 0 Example of the invention 21 2 540 × 1, then 480 × 1 530 × 200 nitrogen gas 10 0 Example of the invention 22 2 540 × 1, then 480 × 1550 × 60  nitrogen gas 10 0 Example of the invention 23 2 540 × 1, then480 × 1 380 × 7200 nitrogen gas 10 8 Comparative example 24 2 540 × 1,then 480 × 1 420 × 1000 nitrogen gas 10 8 Comparative example 25 2 540 ×1, then 480 × 1 450 × 120  nitrogen gas 10 12 Comparative example 26 2540 × 1, then 480 × 1 550 × 20  nitrogen gas 10 7 Comparative example⋆ Asperities of a grained surface were nonuniform and no unetchedportion was able to be determined.

As is clear from the results shown in Table 2, as for the substrates(Sample Nos. 1 to 8 and 16 to 22) prepared on the condition according tothe present invention, in which the homogenization treatment time isshort and the atmosphere of the intermediate annealing is an inert gas,asperities on the roughened surface are uniform and any unetched portionresults even when the alkali etching time is reduced, and therefore, theprocess of the present invention is an excellent process for producing asubstrate. On the other hand, it is clear that as for the substratesproduced through the intermediate annealing conducted at a low treatmenttemperature or for a reduced treatment time (Sample Nos. 9 to 12 and 23to 26), asperities on the roughened surface are uniform but unetchedportions result. The samples of Comparative examples (Sample Nos. 13 to15) in which the atmosphere of the intermediate annealing is air are notsuitable for the substrate since asperities on the roughened surface arenonuniform. Furthermore, no unetched portion was able to be determinedbecause of the nonuniformity of asperities.

As described above, the substrate of the present invention is preparedon the condition in which the homogenization treatment time is short andthe atmosphere of the intermediate annealing treatment is an inert gas.The process of the present invention can exert the effect of realizingan excellent roughened surface on the level better than or equal tothose of known products even when the alkali etching corresponding to apretreatment of the surface-roughening treatment is shortened. Theeffect is also exerted on reducing the treatment time in the entireprocess for producing the substrate and achieving excellentproductivity.

1. A process for producing an aluminum alloy substrate for a lithographic printing plate, the process comprising the steps of: preparing an aluminum alloy ingot having a JIS 1000 series composition; subjecting the aluminum alloy ingot to a homogenization treatment comprising the first stage of holding at 510° C. to 560° C. for 30 minutes to 2 hours and the latter stage of holding at 460° C. to 500° C. for 30 minutes to 2 hours; starting hot rolling and, thereafter, finishing the hot rolling at a temperature of 360° C. or more; conducting cold rolling; conducting intermediate annealing at a heating temperature of X° C. for a holding time of Y sec in an inert gas atmosphere, where X is 400° C. to 620° C. and Y≧2×10⁸×exp(−0.0284X) is satisfied; and conducting final cold rolling.
 2. The process for producing an aluminum alloy substrate for a lithographic printing plate according to claim 1, wherein the aluminum alloy ingot having a composition comprising: 0.1 to 0.50 percent by weight of Fe; 0.03 to 0.30 percent by weight of Si; 0.002 to 0.040 percent by weight of Cu; 0.01 to 0.05 percent by weight of Ti; 0.0001 to 0.02 percent by weight of B; and the balance of aluminum and unavoidable impurities. 